One of the problems that face military platform commanders on a modern battlefield is to positively identify potential targets as being friend-or-foe when one is detected which is within range of a commander's weapon systems. That identification of friend-or-foe targets presents a very difficult decision for a commander of a military platform, such as a tank, who must decide in a split-second as to whether or not a detected target should be engaged while, at the same time, attempting to minimize any possibility of fratricide killing. This problem is rendered even more complex by the proliferation of military equipment which could result in possible conflicts between nations using similar military platforms. Therefore, visual assessments of potential targets is no longer a reliable method of identification for military platforms. Furthermore, technical advances have increased the range and accuracy of weapon systems which increases the difficulty of accurately identifying targets. That difficulty increases with distance, particularly distances which are located near the edge of useful ranges for current identification aids. This problem also exists for air-to-ground combat scenarios as well as ground-to-ground engagements.
No current systems exist which provide reliable, rapid and positive friend-or-foe identification for military platforms on modern land battlefields. Commanders often still rely on low-resolution visual and infrared images to determine if detected targets, be they tanks or other support vehicles, are enemy ones or not. That information may possibly be supported by information derived from a radio network. However, this is not always possible since commanders often have to operate under radio silence in order to avoid being detected by an enemy. In the case of infrared (IR) imagers, the identification of land vehicles is not straight forward even with the most sophisticated thermal viewers. The IR signatures of land vehicles observed by these type of thermal viewers are dependent, to a very large degree, on uncontrollable factors such as the time a vehicle's engine has been running, the time a vehicle has been exposed to direct sunlight, etc. These factors make it difficult to determine if the observed IR signature from a vehicle is that of a friend-or-foe military platform.
Several different types of systems have been previously proposed to perform identification friend-or-foe (IFF) functions, most of which are based on radar or radio technology and a few on electro-optical technology. However, none of these systems have managed to satisfactorily meet the reliability requirements necessary for the type of operations envisaged while, at the same time, remaining covert and secure from observation by enemy forces. Among known IFF systems, some are purely passive and just provide distinctive passive features for interrogation vehicles to identify while others are considered to be active since they have to emit a signal to provide adequate identification. These active IFF systems can be based on a transponder that emits a signal which should only be detectable by a friendly platform and may also involve an interrogator and responder for each platform in order to perform active cooperative identification.
One known IFF technique is for a vehicle to carry a transponder that emits a coded return signal when an interrogating radar pulse is detected by its receiver. U.S. Pat. No. 4,851,849 by Otto Albersdoerfer describes one such active system while another type of IFF system, one which does not require an active transponder, is described in U.S. Pat. No. 4,694,297 by Alan Sewards. The IFF system described in U.S.Pat. No. 4,694,297 only requires an antenna on a target vehicle which can re-radiate or reflect a radar beam and modulate that re-radiated beam in a distinctive manner. This latter system is based on the idea that an illuminating radar source would only detect a small reflected signal from a good antenna which is terminated in a matched load. However, all of the energy intercepted by that antenna will be re-radiated when the antenna terminating impedance provides a short circuit. A substantial reflected signal would then be created which could be detected by the source of the illuminating radar beam. Therefore, an antenna on a target vehicle with a variable termination impedance can modulate a re-radiated radar beam back to the radar source and, as a result, provide an identification signal to the source with that passively reflected radar beam being modulated with the code of the day.
One type of active cooperative electro-optical IFF system is described in U.S. Pat. No. 4,143,263 by K. Eichweber. This system consists of a number of retroreflectors arranged in a circular pattern to obtain omnidirection reception of any interrogating laser beam with a modulator connected in front of each retroreflector. That modulator, for example a liquid crystal, is normally kept in an opaque blocking state and only opened after a coded signal of an interrogating beam has been picked up by a parallel sensor and been decoded to verify it as being a friendly signal. When the modulator is open, the interrogating laser beam is retroreflected back to its source. By varying the light transmittancy of the modulator to selectively mask and unmask the associated retroreflector, an identification coded signal can be added to the retroreflected laser beam to identify the receiver of the interrogating laser beam to the source of that beam.
Another active cooperative electro-optical IFF system is described in U.S. Pat. No. 5,274,379 by R. Carbonneau et al. In this system, each friendly vehicle is provided with a narrow-beam laser transmitter and a receiver with a panoramic detector for a transmitted coded laser beam originating from an interrogating vehicle. If a vehicle detects a coded interrogating laser beam and identifies the code transmitted as being from a friendly source, it provides an unblocking signal to a blocking means surrounding a rotating retro-reflector to clear a transmission path so that the retro-reflector will reflect the interrogating laser beam back to the source where it can be identified by a narrow field-of-view detector of the interrogator. A further modulation is added to the reflected beam to identify the interrogated vehicle as being friendly. If a laser beam is detected from a source without a valid code being identified, the blocking means does not clear the transmission path preventing reflection of that beam by the retro-reflector and a warning is sent to the commander of the vehicle that an unfriendly laser transmission has been detected by the vehicle. Both U.S. Pat. No. 4,143,263 and 5,274,379 reflect an interrogating laser beam back towards the source while adding a further code to the reflected beam.